Polyclonal antibodies are known that react specifically with digoxin, but do not cross-react with steroid hormones (Smith, T. W. et al., Biochemistry 9(2), 331-337, 1970). In addition, monoclonal antibodies are known that have strong affinity for digoxin (Hunter, M. M. et al., J. Immunol. 129(3), 1165-1172, 1982). Moreover, it is also recognized that monoclonal antibodies to digoxin demonstrate a strong neutralizing action against digoxin effects in guinea pig atrium (Wong, P. C. et al., Eur. J. Pharmacol. 136, 437-440, 1987). However, there are no detailed findings concerning the site that is recognized by these monoclonal antibodies in terms of the structure of digoxin.
Note, the inventors of the present invention have produced polyclonal antibodies to ouabain and reported that they react with cardiac glycoside, especially ouabain, digoxin, and endogenous ouabain-like substances (Masugi, F. et al., Biochem. Biophys. Res. Commun. 135(1), 41-45, 1986).
Although antibodies having clearly defined specificities are required for the accurate measurement of blood concentrations of cardiac glycosides in various forms of research and clinical applications of cardiac glycosides, as was previously stated, such antibodies are presently not known. Thus, the present invention provides monoclonal antibodies that specifically react only with cardiac glycoside having a specific structure.